Nowadays, power suppliers have been commonly applied to various electronic products, and owing to the developed technology, the output power thereof is gradually increased. However, for further raising the power, it has to increase the operation efficiency. Generally, the efficiency of power supply is referenced to a power factor, which represents the relationship between the effective power and sum power consumption (apparent power), that is, the rate of effective power to sum power consumption, so that the closer the effective power to the sum power consumption, the higher the power factor, namely, the better the operation efficiency. Therefore, the improvement of operation efficiency can achieve the advantages of real output power increment, consumption reduction and waste heat decrement. And, also because the safe regulation and the harmonic wave regulation issued by European Union request a high standard of the power consumption efficiency for all electronic products, all manufacturers are devoted to improving the operation efficiency of products. Currently, the power supply industry commonly utilizes a PFC (Power Factor Correction) circuit to increase the power factor, and in particular, an active PFC circuit can increase the operation efficiency up to 80%. The working theory of the active PFC circuit is that the phase of an input voltage is detected, and a semiconductor switch unit (generally, a MOSFERT) and a PWM (Pulse Width Modulation) unit are utilized to control and regulate the phase of current to be more closer to the phase of the voltage, so as to increase the real power. The conventional circuit is shown in FIG. 1, after passing through a rectifier unit 1 and a primary filter unit 2, the inputted AC power is transformed into a DC power, which is then transmitted a back end power output unit 3. And, between the filter unit 2 and the power output unit 3, a PFC circuit is disposed which includes a switch unit 4, an energy storage inductor 8, an output diode 9, a sensing resistor 5 and a PWM unit 6. Through the switch unit 4 regulating the phase of the DC current outputted by the rectifier unit 1, the PWM unit 6 can generate the duty cycle for the switch unit 4, and then, the PWM unit 6 utilizes a reference voltage to generate the conductive cycle for the switch unit 4. Conventionally, the method to obtain the reference voltage is to series connect the sensing resistor 5 with the switch unit 4, so that the sensing resistor can generate a reference voltage when the DC current passes therethrough, thereby the PWM unit 6 can adjust the phase of the DC current. However, since the output power of the power supply is gradually increased and also the outputted DC current, the consumption of the sensing resistor 5 in the conventional PFC circuit will become higher, too. The equation for calculating the power is as followed:P=I2×R For making the PWM unit 6 to identify the reference voltages at two ends of the sensing resistor 5, one method is to utilize a PWM unit having a higher sensitivity. However, this may increase the cost of the PWM unit. Alternatively, a larger sensing resistor 5 can be used, so that the reference voltages generated at two ends thereof can be big enough to be correctly detected by the PWM unit 6. But, this method causes the consumption of the PFC circuit which nevertheless causes the product fail in safe regulations. Consequently, a method with reasonable cost and low consumption should be needed.